This invention relates to a method and apparatus for separating air.
Air is separated commercially by rectification. The most frequently used air separation processes include the steps of compressing a stream of air, purifying the resulting stream of compressed by removing water vapour and carbon dioxide therefrom and cooling the stream of compressed air by heat exchange in a main heat exchanger with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising two rectification columns, one operating at higher pressures than the other, a top region of the higher pressure rectification column being in heat exchange relationship with a bottom region of the lower pressure rectification column. Most or all of the cooled air is introduced into the higher pressure rectification column and is separated therein into oxygen-enriched liquid air and nitrogen vapour. The nitrogen vapour is condensed in a condenser-reboiler. A part of the resulting condensate is used as liquid reflux in the higher pressure rectification column. Oxygen-enriched liquid air is withdrawn from the bottom of the higher pressure rectification column, is sub-cooled, and is introduced into an intermediate region of the lower pressure rectification column through a pressure-reducing valve. This oxygen-enriched liquid air is separated into oxygen and nitrogen products in the lower pressure rectification column. These products may be withdrawn in the vapour state from the lower pressure rectification column and form the returning streams against which the incoming air stream is heat exchanged.
Liquid reflux for the lower pressure rectification column, is provided by taking the rest of the liquid nitrogen condensate, sub-cooling it, and passing the resulting sub-cooled liquid into the top of the lower pressure rectification column through a pressure reducing valve.
Conventionally, the lower pressure rectification column is operated at pressures in the range of 1 to 1.5 bar. At such pressures it is desirable to use liquid oxygen at the bottom of the lower pressure rectification column to meet the condensation duty at the top of the higher pressure rectification column. Sufficient liquid oxygen is evaporated thereby to meet the requirements of the lower pressure rectification column for reboil and to enable a good yield of gaseous oxygen product to be achieved. It is known however that the yield of oxygen can deteriorate if changes are made to the operating conditions of the lower pressure rectification column. For example, with increasing operating pressures in the lower pressure rectification column, and hence in the higher pressure rectification column as well, the yield of oxygen becomes progressively lower. Such a reduction in the yield of oxygen can be attributed to a relative lack of liquid nitrogen reflux in the lower pressure rectification column. According to EP-A-0 384 688, the liquid nitrogen reflux from the higher pressure rectification columns may be supplemented by taking a part of the nitrogen product from downstream of its heat exchange with the incoming air, compressing it, passing the compressed nitrogen back through the main heat exchanger cocurrently with the incoming air, and condensing the cooled, compressed nitrogen by heat exchange with a part of the oxygen-enriched liquid air. This modification of the air separation process has however a limited efficiency and requires additional compression machinery.
The method and apparatus according to the present invention relate to a different approach to addressing the problem of compensating for any shortage of liquid reflux in the lower pressure rectification column.